1. Field of the Invention
The present invention relates to an antenna device used in a radio frequency IC device such as an RFID device that performs contactless communication by near-field transmission and a radio frequency IC device including the antenna device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-010264 discloses a contactless IC card used as an RFID device. FIG. 1 illustrates an equivalent circuit of the contactless IC card disclosed in Japanese Unexamined Patent Application Publication No. 2001-010264. This contactless IC card performs contactless communication with a reader/writer. A parallel resonance circuit is provided for a radio frequency IC chip 11. The parallel resonance circuit includes an inductor L provided by an antenna coil 13, a resistor R for the entire circuit, an adjusting resistor 14, a capacitor C for the entire circuit which has a capacitance based on the IC chip and a stray capacitance occurring in the circuit, and an adjusting capacitor having a capacitance Cad.
In order to obtain a good state of communication, the sharpness (Q) of the resonance circuit is controlled by controlling a resistance value Rad of the adjusting resistor 14 included in the resonance circuit and a resonance frequency is controlled by the capacitance Cad of an adjusting capacitor 15.
When such an IC card moves closer to a reader/writer so as to communicate with the reader/writer, however, antenna coils included in both of them are coupled and the inductance values of the antenna coils are changed. As a result, the resonance frequencies of the resonance circuits including the antenna coils are changed and gains are significantly changed.
FIGS. 2A-2C are diagrams illustrating the above-described situation. A resonance circuit 31 including an antenna coil Lr and a capacitor Cr is provided in a reader/writer 300. A resonance circuit including an antenna coil La and a capacitor Ca is formed in a radio frequency IC device 200, and is connected to a radio frequency IC 21.
An S11 characteristic (return loss) is represented by a characteristic curve Ra in FIG. 2C. The S11 characteristic is an S-parameter and is obtained when the antenna device is observed from the radio frequency IC 21 included in the radio frequency IC device 200. The return loss reaches its peak at a frequency foa when the radio frequency IC device 200 is located at an appropriate distance from the reader/writer 300.
On the other hand, as illustrated in FIG. 2B, if the radio frequency IC device 200 is excessively close to the reader/writer 300, the antenna coil La included in the radio frequency IC device 200 and the antenna coil Lr included in the reader/writer 300 are magnetically coupled and the inductances of both of them are increased. Accordingly, as illustrated in FIG. 2C using a characteristic curve Rb, the return loss reaches its peak at a frequency fob lower than the frequency foa.
Under the above-described condition in which the antenna included in the radio frequency IC device 200 and the antenna included in the reader/writer 300 are coupled by near-field transmission, as both of them get closer to each other, the resonance frequencies of both of the antennas are shifted in the direction of a lower frequency. If this antenna resonance frequency is lower than a frequency used by the radio frequency IC device 200 (that is, a communication frequency represented by a symbol fs in FIG. 2C), the antenna coil cannot function as an inductor and an antenna gain is significantly reduced. Consequently, communication cannot be performed.
In the conventional art, in order to prevent the resonance frequency from being lower than the communication frequency even if the resonance frequency is shifted in the direction of a lower frequency, it is required that the resonance frequency of the antenna device be set to a frequency that is 10 to 20 percent higher than the communication frequency in advance. Furthermore, in order to make communication possible even under the condition in which the resonance frequency is higher than the communication frequency, it is required that the value Q of a resonance circuit including an antenna coil be set to a low value by disposing a resistor in the resonance circuit as illustrated in FIG. 1.
However, under the conditions allowing the radio frequency IC device 200 to communicate with the reader/writer 300 even if they are excessively close to each other, a resonance frequency of an antenna of the radio frequency IC device 200, which is obtained when the distance therebetween is normal, is shifted to a direction of a frequency higher than the communication frequency. Accordingly, if the distance between the radio frequency IC device 200 and the reader/writer 300 is larger than the normal distance, antenna gains are significantly reduced. Consequently, a sufficient communication distance cannot be obtained.
Furthermore, when the value Q of the resonance frequency including an antenna coil is set to a lower value, a relatively stable gain can be obtained with the broad characteristics of the resonance circuit even if the resonance frequency is shifted. On the other hand, however, since the value Q is set to a lower value, a gain is lowered regardless of the distance between the radio frequency IC device and the reader/writer.